High Speed Downlink Packet Access (HSDPA) multipoint transmission, in which multiple cells are capable to transmitting possibly different data to the same User Equipment (UE) simultaneously using the same frequency, was approved as a new study item (SI) at Third Generation Partnership Project (3GPP) RAN #50. Possible multipoint transmission methods that should be evaluated according to the SI include:                Switched transmit diversity techniques (e.g., HS-DDTx)        Multi-flow transmission techniques (e.g. SF-DC-HSDPA)        Single frequency network transmissions (e.g. HS-SFN multipoint)        
Furthermore, according to the SI both intra and inter-NodeB coordination are being considered. The following distinction between intra and inter-site coordination can be made:                Intra-site coordination: All cells wherefrom data can be transmitted to a UE are controlled by one logical baseband unit. Note that the baseband unit can be implemented in a way so that its functionality is distributed across multiple cells (possibly at different location) but where direct communication between the different parts of the functionality is supported by means of, e.g., some proprietary interface. This means that information for the cells can be shared without involving a central node such as a Radio Network Controller (RNC).        Inter-site coordination: Cells wherefrom data can be transmitted to a UE are controlled by multiple logical baseband units where the different baseband units only can share information with each other by means of sending it via a central node, such as an RNC.        
Another practical example of intra-site scenario is Main Unit—Remote Radio Unit (MU-RRU) solutions in which multiple cells with antennas at different physical locations share base band unit.
For multi-point transmission techniques, transmission of data can take place from multiple different cells to one UE at the same time. The set of cells wherefrom transmission can take place is here referred to as a configured CoMP set. In case of intra-site coordination the baseband unit controlling the cells in the CoMP set can get information for all available cells without involving the central node such as an RNC. Amongst the cells in the CoMP set a logical entity can at a given time select a subset of the cells wherefrom actual transmission takes place. This will be referred to as a selection process, i.e. how to select the secondary High-Speed Downlink Shared Channel (HS-DSCH) serving cells, as CoMP set selection. This selection process results in a selected CoMP set.
It has been argued that the determination of the secondary serving HS-DSCH cell is linked to legacy mobility procedure. For example, the network may need to sort active set cells based on pilot strength measurements reported by the UE and determine the second strongest cell as the secondary serving HS-DSCH cell. Furthermore, depending on whether the network supports SF-DC-HSDPA (Single Frequency Dual Cell High Speed Downlink Packet Access) across non-co-located sectors, it may need to restrict the determination to cells that belong to the same NodeB as the serving HS-DSCH cell.
As noted above it has been proposed that existing mobility events are used for selecting the CoMP set. In this case the configured and selected CoMP set would be identical. Further if only the existing mobility events are (re)used for selecting the CoMP set this needs to be performed by the RNC (since mobility events are signaled via Radio Resource Control (RRC) between the UE and the RNC).
There is a constant demand to improve the performance of cellular radio systems.
Hence there exists a need for new methods and devices providing improved selection of CoMP set.